RuggedCircuits:
This doesn't look like a "buyer charge", just a notice from PayPal that your credit card company may charge you (independently), most likely because you are making a payment in non-USD funds?Having said that, PayPal definitely tries to steer people away from paying with credit cards and instead paying directly through them. If you are sure it's a USD transaction then I'd brush it off as a "marketing" tactic.
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The DIN Rail Mount kit for Arduino: quickly attach your Arduino to standard DIN rail
I would rather find out if this is a tactic like you said to make more money for paypal 
a bit evil isn't it? I'm asking the seller to find out their currency. May even consider to try the credit card and pay the $4 to find out if paypal is lying. Small price to pay to find out some truth.